The correct answer is C. Create a validation rule that uses the PRIORVALUE function.
The requirement is: once Estimated Hours is set, users can decrease the value but cannot increase it. A validation rule can compare the new value with the old value by using PRIORVALUE().
Example logic:
Estimated_Hours__c > PRIORVALUE(Estimated_Hours__c)
Salesforce documents PRIORVALUE() as the function used to return the previous value of a field, and Salesforce examples use it in validation rules to compare a changed value against the prior value.
A is incomplete because ISCHANGED() only detects that the field changed; it does not determine whether the value increased or decreased.
B is incorrect because a default value does not control later edits.
D is incorrect because PREVGROUPVAL() is used in reports, not validation rules.

The correct answer is C.
The app builder needs to display only the first five digits from the parent Account's ZIP+4 custom text field. Salesforce's LEFT() function returns a specified number of characters from the beginning of a text string, so the correct formula is:
LEFT(Account.Zip_Code__c, 5)
Salesforce formula documentation confirms that LEFT returns characters from the beginning of a text string.
BEGINS() checks whether text starts with specific characters. LPAD() pads text to a specified length. TEXT() converts certain data types to text, but it does not extract the first five characters.

The correct answer is A. Account Object.
Roll-up summary fields are created on the parent object, because the summary value is displayed on the parent record. Here, the KPI values need to appear for each Account, and they summarize related Opportunity records. Therefore, the app builder should go to the Account object in Object Manager and create roll-up summary fields there.
For this requirement, the app builder would create separate roll-up summary fields on Account, such as:
Count of open Opportunities
Count of won Opportunities
Salesforce documentation describes roll-up summary fields as fields that calculate values from related records, such as records in a related list.
C . Opportunity Object is incorrect because Opportunity is the child/related record being counted.
D . Lightning App Builder is incorrect because Lightning App Builder controls page layout/components, not the creation of roll-up fields.
B . Lightning Object Creator is used to create objects from spreadsheets, not build rollups.

The correct tool is Data Loader.
Cloud Kicks needs to insert over 1 million net new Account records. Salesforce Data Loader is the correct tool for large-volume imports because it supports bulk insert, update, delete, and export operations. Salesforce's Data Loader documentation describes it as a client application for bulk data import and export, including inserting Salesforce records. Salesforce also distinguishes it from the Data Import Wizard, which is limited to smaller imports.
Why the other options are wrong:
Data Import Wizard is not suitable for over 1 million records because it is intended for much smaller imports. Schema Builder is for viewing and creating data model components, not loading a million records. Lightning Object Creator creates objects from spreadsheets, not mass-inserting large Account datasets into an existing model.

The correct answer is C. Two Master-detail relationships on a new custom object.
This is a many-to-many relationship: one Contact can be related to many Solar Projects, and one Solar Project can be related to many Contacts. Salesforce's standard declarative design for many-to-many relationships is a junction object. A junction object is a custom object with two master-detail relationships connecting the two related objects.
A is incorrect because the two master-detail relationships belong on the new junction object, not directly on Contact and Solar Project.
B can technically relate records, but it is not the standard Salesforce many-to-many model tested here.
D does not correctly create a scalable many-to-many model.